Particulate blasting is widely used for deburring and cleaning cast and machined metal parts, for cleaning surfaces of these objects, and for surface preparation of metals. In this blasting process, ferrous particles such as steel grit or steel shot, nonferrous metal particles, artificial abrasive particles such as alumina, and natural abrasive particles such as silica sand, are generally used as the blasting media. Such blasting media, however, can be successfully utilized only when working with hard or heavy metal objects.
When parts are made of soft metals such as copper or zinc, or parts are formed of synthetic resins, these known hard blasting particles can not be utilized for blasting of the parts since such hard blasting media excessively deform the objects and/or abrade the surface thereof to an undesired degree. Accordingly, for these softer parts, the blasting normally involves the use of so-called "soft grits". This "soft grit" is mainly natural grit made from plant seeds or walnut shells.
In comparing the soft and hard grits or particles used in blasting, it is generally recognized that the blasting media required for use on hard or heavy metal parts must generally have substantial mechanical properties such as mechanical strength, surface hardness and impact resistance, and for this reason the hard particles of the type mentioned above are commonly used. On the other hand, when the object to be blasted is formed from soft metals or synthetic resins, while the soft blasting media must have many of the same properties as the hard particles, nevertheless it is also a requisite that the soft blasting media not abrade the surface of the article too harshly, or not change the surface condition of the article too severely.
For example, when the object comprises an IC chip (integrated circuit chip), a metal lead frame is packaged within a housing of synthetic resin which is molded therearound, which resin is positioned such that the metal conductive leads project outwardly from the resin housing at selected locations. Due to this molding of the resin housing about the lead frame, resin burr or flash is conventionally formed at the interface between the resin housing and the metal lead frame. Resin smears also normally occur on the metal leads, or between the metal leads, where they project outwardly from the resin housing. In the past, it has been conventional practice to remove the resin burrs, flash and smear by blasting the IC chips with alumina particles of No. 120 to No. 200 mesh size. While this known technique has been able to successfully remove the resin burr or flash or smear, nevertheless this at the same time undesirably abrades the surface of the resin package, thereby significantly affecting the appearance and hence the commercial value of the resultant product. Further, in situations where the resultant product must possess a precise surface condition or finish, this abrading of the surface is wholly undesirable and can make the part wholly unacceptable for its intended use.
To overcome the above problem, parts or objects of this type have hence been blasted using soft grit so as to avoid excessive surface abrasion. However, the natural soft grits such as corn cob, walnut shell and cherry seed, when mixed with water to form a slurry and then sent to a blasting gun, have proven generally unsatisfactory in performance since the particles swell and are not capable of withstanding this type of usage.
Attempts have been made to utilize blasting media formed as synthetic resin particles. However, in order for the blasting media to perform satisfactorily, the media must have sufficient density to create a substantial impact against the object, but at the same time the media must have properties such that it does not hurt or abrade the surface of the article. These two seemingly contradictory requirements hence appear to demand the use of media which has substantial weight but which still has a soft surface. While synthetic resin particles are presently being used, as noted above, nevertheless these synthetic resin particles have also been unable to perform satisfactorily, and hence unable to solve the apparent contradictory requirements for media usable with resin-metal parts, such as IC chips. As to the synthetic resin particles presently being used as blasting media, such particles normally are rather large in diameter, such as a mean diameter of between 0.5 to 2.0 mm. When such particles are used for removing resin flash and burrs, such as from an IC chip, the particles can only partially remove the burr or flash and hence are incapable of finely finishing the complete part.
Further, when these known synthetic resin particles are used in a wet blasting process, it has been observed that the particles do not uniformly mix within the stream of blasting water. For example, the conventional plastics used for these synthetic resin particles are styrene, and polyamide, which plastics have a specific gravity in the range of about 1.0 to 1.1. Other known resins, specifically engineering plastics such as polycarbonate (specific density 1.3) and polyacetyl (specific gravity 1.4) are somewhat heavier but, by themselves, also do not perform a satisfactory finishing of the articles as regards complete removal of the burr, flash and smear. When these conventional resin particles are utilized in conjunction with a stream of water for wet blasting, which is believed to be a preferred technique for the blasting of articles such as IC chips, it has been observed that the plastic particles do not readily sink in water and hence tend to float within the stream, so that a uniform mixture of the particles within the water stream is not achieved, and hence uniform blasting of the article or object can not be effected.
With respect to blasting particles formed of polycarbonate, attention is directed to U.S. Pat. No. 3,313,067 which relates to a process for deflashing articles utilizing polycarbonate resin particles for blasting the formed parts or articles, which particles have a mean diameter of from about 25 mils to about 200 mils (about 5/8 mm to about 8 mm). The blasting process of this patent possesses the disadvantages discussed above.
Thus, it is an object of this invention to provide an improved fine blasting media specifically for wet blasting of parts and articles having resin or other relatively soft portions and surfaces so as to permit efficient and effective removal of burrs, flash and the like without damaging, scarring or seriously abrading the surface of the part. This invention also relates to the method of using and manufacturing this improved blasting media.
More specifically, the improved blasting media involves fine particles having a mean diameter less than 0.5 mm, and a mean diameter preferably between 0.2 mm and 0.3 mm, with the specific gravity of the particles being greater than 1.5. The particles are irregular, polygonal shaped, hard particles of a plastics resin having a specific gravity of about 1.3 or greater, such as polyacetyl or polycarbonate, with one or more glass beads being embedded within and surrounded by the resin to provide the particles with the desired density. The resulting particles, due to their size and density, readily uniformly mix with water to permit uniform blasting of articles, and at the same time the plastic resin acts as a coating for preventing the glass beads, which provide the particles with the desired density, from damaging the surface of the part being blasted. The plastics resin itself possesses substantial strength to perform the desired impacting and removing of the burr and flash from the object without scarring or abrading the surfaces of the part. This improved blasting media includes about 85% to about 70% by weight of plastics resin, and about 15% to about 30% by weight of glass beads.
While the coating of glass beads with a resin for forming blasting particles is already known, as disclosed in U.S. Pat. No. 3,225,495, nevertheless the particles in this latter patent utilize an epoxy resin for forming a thin coating around each glass bead, which resin is normally filled with polishing particles. The thus-formed particles are basically spherical, and have a single glass ball forming the core, and hence the major mass of the particle. In fact, only about 2 to 12 parts by weight of resin are used for each 100 parts by weight of glass beads. These particles are heavy and hence severely impact the surface of the articles and normally create a compressive stress layer at the surface. The blasting particles of this latter patent hence are suitable for polishing, but are not suitable for effective removal of burrs and flash without causing substantial surface damage. Further, the coating on the glass beads is primarily for the purpose of preventing fracturing of the glass beads upon impacting thereof against a surface.
In the improved method of making the blasting particles according to the present invention, the glass beads are coated with a silane coupling agent and thereafter are mixed with the selected plastic resin. The mixture is then extruded into selected shapes, such as rods, which are then pelletized. The pellets are then frozen rapidly at a very low temperature, and are immediately crushed or pulverized when in this fragile frozen state so as to form the fine but irregular blasting particles.
Other objects and purposes of the invention will be apparent after reading the following specification.